Gram-positive bacterial infections are widespread and represent a significant proportion of iatrogenic and other infections prevalent in hospitals and medical facilities. The inevitable development of antibiotic-resistant strains necessitates the continuous development of new means of controlling these infections. Thus, there is a need in the art for methods and compositions useful in treating Gram-positive bacterial infections in animals and humans (Waldvogel, in Principles and Practice of Infectious Diseases, Mandell et al.,eds., John Wiley, New York, 1995, pp 1754-1775).
In mammals, inflammation is a complex response that involves, at a minimum, the recruitment of immune cells, changes in vascular permeability, and the accumulation of extracellular fluid containing a wide variety of bioactive compounds, including proteins and peptides. Among the bioactivities expressed in such inflammatory fluids are antibacterial activities that are directed against both Gram-positive and Gram-negative bacteria.
In rabbits, sterile injection of glycogen into the peritoneal cavity induces the formation of a cell-rich exudate consisting of polymorphonuclear leukocytes (PMNs) and a cell-free (ascitic) fluid (designated AF). In this model system, potent antibacterial activity accumulates in AF against both Gram-negative and Gram-positive bacteria. The activity against Gram-negative bacteria is attributable to two PMN-derived antibacterial proteins acting synergistically, the Bactericidal/Permeability-Increasing Protein (BPI) and the pl5s (Weinrauch et al., J. Clin.Invest. 95:1916, 1995). The Gram-negative bactericidal activity of AF is abolished by anti-BPI serum, indicating an absolute dependence on BPI. Antibacterial activity in AF against Staphylococci, by contrast, is unaffected by anti-BPI serum, indicating that BPI is not responsible for Gram-positive killing.
The present inventors have unexpectedly discovered that the Gram-positive bactericidal activity of rabbit AF is due to Type II phospholipase A2 (PLA.sub.2 (II) that is present in the exudate. The bactericidal activity of PLA.sub.2 (II) is independent of other constituents and is specific to Gram-positive bacterial targets. Furthermore, Gram-positive bactericidal activity has been found to be a property of all tested members of the PLA.sub.2 (II) enzyme family, such as those derived from other tissues and mammalian species. Prior to the present invention, no independent bactericidal activity expressed in biological fluids (including unmodified plasma and inflammatory fluids) was ascribed to this protein. The present invention thus provides methods and compositions for killing Gram-positive bacteria that are applicable to many different therapeutic applications and that provide an important therapeutic modality to treat antibiotic-sensitive and antibiotic-resistant Gram-positive bacterial infections.